Low speed and low current clock oscillators are widely used in electronics. They provide a time base for real time clocks (RTC's) for keeping the time, timing for periodic operations, etc. They can provide a wakeup signal for devices that are in sleep mode for power saving. In low power operations, if the product is in power saving mode most of the time, the current consumption of the oscillator that must be on all the time is critical. Usually oscillators are based on crystal or RC. Oscillator circuits produce repeating waves of various shapes. Every oscillator has timing components that determine the oscillator's frequency. An RC oscillator uses a resistor and capacitor to perform this function.
Currently available low power oscillators are not fully contained in a standard VLSI chip. They require external components. The crystal oscillator requires at least a crystal and the RC oscillator requires external resistor or capacitor or both. An external resistor or capacitor is needed because the on-chip components are not accurate. Also, it is difficult to produce large on-chip capacitors or large on-chip resistors, and they consume excessive chip surface area.
Calendaring or docketing systems have been devised to track expiration dates of various articles. With the advent of computers, such systems have become more prevalent. However, the data in such systems must be constantly updated and maintained; otherwise the output from such systems becomes unreliable. In some applications, such as pharmacy operations, it would be a tremendous burden to constantly update and maintain a system for tracking expiration dates for every drug in inventory.
As an alternative to calendaring systems, for some time now, the idea has been known that if a label could provide some visual signal that the expiration date has been reached, the burden of monitoring time-sensitive or perishable inventory could be reduced. A timer and storage condition indicator configured as a label to be affixed to a medicine bottle. Once the shelf-life of the medicine in the bottle has expired, the word “Expired” appears on the label.
Many inventors and companies have tried to design such a label. One option was by use of a particular carrier liquid and black dye. The timing function is achieved by a predetermined migration rate of the carrier liquid through a porous material. The disadvantage of this technique or any chemical technique is that it is limited to short time and it is very inaccurate. Moreover, it has to be redesigned for any new duration to measure. Other inventors suggested to use an electronic circuit that will count the time and a LCD screen to show the result. However, no such design was till now made thin enough, flexible enough and cheap enough to be a viable solution.
The technology for manufacturing very thin and flexible batteries or even batteries printed on a label exists. It is also possible to attach a chip to the label and this label can be manufactured in high volumes in low cost. Eidelson, in U.S. Pat. No. 6,337,836, teaches a programmable electronic reminder device for providing a signal at a selectively predetermined date and time includes a programmable real time clock circuit, a thin, sheet-like battery, a light emitting polymer for illuminating a predetermined alert message and a programming interface mounted to a pliant substrate in a layered arrangement. However, Eidelson's device cannot be used to make a practical device, and is not configured for low cost.
It is not desired, however to include any additional external component to the label. It is one of the aspects of the current invention to provide a low power oscillator and is entirely confined inside the chip.
Low power is essential for battery powered devices, such as when the battery is very small, or if the battery is a printed battery. Such batteries may have a power capacity of about 1 mAh. Where the intended lifespan of the device is 5 years, the allowed consumption is 23 nA. It is another aspect of the current invention to provide an oscillator that can reliably operate at such low current consumption, on the order of 10-100 nA.
Transistors are like switches. When turned on, metal-oxide semiconductor (MOS) transistors allow the current to flow through them and when turned off do not allow current to flow through them. The current is controlled by the VGS voltage and there is a voltage threshold, such that if VGS is larger than the threshold voltage the current is flowing and if VGS is far below the threshold the current is shut off. In case the controlling voltage is slightly below the threshold, the transistor is not shut off entirely and a very small current is flowing. The operation of the transistor is the sub-threshold region is highly dependent on the temperature, the process and the layout and is susceptible to noise. Therefore, without continuous online calibration, a current source in the subthreshold region is highly inaccurate.
When the resistor and capacitor are small, the time constant is short and the oscillator must operate at high frequency, hence drawing a lot of power. Furthermore, the current drawn by the oscillator is on the order of VDD/R. Since the largest reasonable on-chip resistor is 1 Mohm, the minimum current drawn by on chip RC oscillator will be a few uA's.
Kwon, in U.S. Pat. No. 7,498,884, teaches a self calibrating RC oscillator. This oscillator contains an RC oscillator and a ramp circuit, and a close loop circuit to control the frequency of the RC oscillator according to the ramp circuit. The ramp circuit, in turn is controlled by a resistor and capacitor. The resistor is temperature stable and the capacitor can be trimmed by switches such that the oscillation frequency is equal to a reference crystal oscillator.
Ng et al in U.S. Pat. No. 7,705,685 teaches a low voltage oscillator with capacitor ratio selectable duty cycle. This oscillator is using MOS transistors in their sub-threshold mode it is designed to be able to work in low voltage and the capacitor ratio is used for duty cycle selection. However, Ng device does not teach how to use capacitor ratio to provide low current operation with stable frequency and current consumption.